What Mobile Roaming Is
Roaming simply means using your UK SIM on a foreign network when you travel. Your phone connects to a partner network abroad, and your calls, texts and data are routed back through your UK provider. It is what lets you use your phone the moment you land without buying a local SIM.
The catch is cost. When you roam, your provider pays the foreign network for your usage, and whether that cost is passed on to you depends on where you are and what your plan includes. The rules differ sharply between the EU and the rest of the world — and even within the EU, there are caps and fair-use limits to understand.